Corundum, structure

The growth of vanadium oxide overlayers on Rh(l 11) converges after a number of intermediate stages to the formation of a three-dimensional bulk-like epitaxial V203 film [90], which is oriented with the (0 0 01) plane of its corundum structure parallel to the Rh(l 1 1) substrate surface. The V203 phase is the thermodynamically stable... 

X-ray diffraction allows the dimensions of the unit cell to be accurately measured. If the structure type of the material is known, the ideal cell contents are also known. Thus, the unit cell of a crystal of composition M2O3 that adopts the corundum structure contains 12 M atoms and 18 O atoms (Supplementary Material, SI). This readily allows the theoretical density of a solid to be calculated. The weights of all of the atoms in the cell are added, and this is divided by the cell volume. 

The presence of relatively small Cr3 1 cations and the close-packed corundum structure anions suggest that Schottky defects will be preferred. Oxidation will add anions and reduction will remove anions. The four equilibria can now be written. 

The gA1203, hR30, structural type (corundum structure) is ... 

Mixed (Fe,Mn)203-bixbyite structure, black (Fe.Mn)203-corundum structure, brown ... 

In some cases, mixed valence states may appear simultaneously on two different atoms because of an overlap of the redox energies associated with the two atoms. In the ilmenite FeTi03, the valence states Fe " and Ti are stabilized with an ordering of Fe and Ti on crystallographically inequivalent sites FeV03, on the other hand, has the corundum structure with Fe and V randomly distributed on equivalent sites with the valence states Fe and This observation places the top of the Fe " 3d band (level) below the bottom of the Ti 3 d band, but above the top of the 3 d band, so charge transfers of the type... 

The ilmenite structure is adopted by oxides of formula ABO3 when A and B are similar in size and their total charge adds up to +6. The structure is named after the mineral of Fe Ti" 03, and the structure is very similar to the corundum structure described... 

This material, which has the corundum structure, is a semiconductor at low temperatures, the optical band gap being 0.2 eV (Lucovsky et al. 1979). We should probably consider it to be an intrinsic semiconductor, but the activation energy in the conductivity does not appear to be constant. The thermopower (Chandrasekhar et al. 1970) is about 900pVK 1 at 100K and 500pVK 1 at 200 K this would suggest an activation energy of about 0.06 eV, or less than half the band gap.f This makes it likely that one of the carriers is a small polaron the... 

The structure of the insulating low-temperature phase is a monoclinic distortion of the corundum structure. Structure determination (Dernier and Marezio 1970) shows that the V-V distance increases from 2.697 to 2.745 A, which across shared octahedral edges increases from 2.882 to 2.907. The average V-O distance remains constant. The material is antiferromagnetic, with a moment of 1.2/ B on... 

Fig. 6.10 Lattice parameters c and a for V203 with corundum structure (McWhan and...

Unfortunately, any attempt to reduce ferric oxide results in the formation of magnetite as a distinct separate phase, and there is no solubility of this spinel in the corundum structure ( 5, 6). Thus, all the properties reported above for were... 

Table III. PCM modelization of 170 efg tensors in diopside CaMgSi206 and forsterite Mg2Si04.Units for Vaa are V.A"2 and 1 - yj = 6.3 as for the corundum structure.

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